Oxide semiconductor film, transistor, and semiconductor device

ABSTRACT

To provide an oxide semiconductor film which has high stability and does not easily cause variation in electric characteristics of a transistor, a transistor including the oxide semiconductor film in its channel formation region, and a highly reliable semiconductor device including the transistor. The oxide semiconductor film including indium includes a crystal part whose c-axis is substantially perpendicular to a surface of the oxide semiconductor film. In the crystal part, the length of a crystal arrangement part containing indium and oxygen on a plane perpendicular to the c-axis is more than 1.5 nm. Further, the semiconductor device includes the transistor including the oxide semiconductor film in its channel formation region.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an oxide semiconductor film, atransistor, a semiconductor device, and manufacturing methods thereof.

In this specification, a semiconductor device refers to all types ofdevices which can function by utilizing semiconductor characteristics;an electro-optical device, a semiconductor circuit, and an electronicappliance are all semiconductor devices.

2. Description of the Related Art

Attention has been focused on a technique for forming a transistor usinga semiconductor thin film formed over a substrate having an insulatingsurface (also referred to as thin film transistor (TFT)). The transistoris applied to a wide range of electronic devices such as an integratedcircuit (IC) or an image display device (display device). Asilicon-based semiconductor material is widely known as a material for athin semiconductor film applicable to a transistor.

Further, recently, an oxide semiconductor has been attracting attentionas a material of a thin semiconductor film. For example, a transistorwhose semiconductor layer including an amorphous oxide containing indium(In), gallium (Ga), and zinc (Zn) (an In—Ga—Zn—O-based amorphous oxide)is formed over a substrate is disclosed (see Patent Document 1).

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2011-181801

SUMMARY OF THE INVENTION

Improvement in reliability of semiconductor devices is important forcommercialization of the semiconductor devices. Variation and decreasein electric characteristics of the semiconductor devices mightparticularly reduce the reliability thereof.

The stability to heat, light, or the like of a semiconductor film usedfor a channel formation region of a transistor significantly influencesvariation in electric characteristics of a semiconductor device.

An object is to provide an oxide semiconductor film which has highstability and does not easily cause variation in electriccharacteristics of a transistor.

Another object is to provide a transistor which includes the oxidesemiconductor film in its channel formation region and has electricallystable characteristics.

Another object is to provide a highly reliable semiconductor deviceincluding the transistor.

In the invention disclosed in this specification, an oxide semiconductorfilm is an oxide semiconductor film including a crystal part whosec-axis is substantially perpendicular to a surface of the oxidesemiconductor film, i.e., a c-axis aligned crystalline oxidesemiconductor (CAAC-OS) film. In this specification, an oxidesemiconductor film including a crystal part whose c-axis issubstantially perpendicular to a surface of the oxide semiconductor filmis referred to as CAAC-OS film.

One embodiment of the invention disclosed in this specification is anoxide semiconductor film including indium, which includes a crystal partwhose c-axis is substantially perpendicular to a surface of the oxidesemiconductor film and in which the length of a crystal arrangement partcontaining indium and oxygen on a plane perpendicular to the c-axis inthe crystal part is more than 1.5 nm.

Another embodiment of the invention disclosed in this specification isan oxide semiconductor film including indium, which includes a crystalpart whose c-axis is substantially perpendicular to a surface of theoxide semiconductor film and in which the length of a crystalarrangement part containing indium and oxygen on a plane perpendicularto the c-axis in the crystal part is more than or equal to 2 nm and lessthan or equal to 20 nm.

Another embodiment of the invention disclosed in this specification isan oxide semiconductor film including indium, which includes a pluralityof crystal parts. C-axes of the plurality of crystal parts are aligned.Each of the c-axes is substantially perpendicular to a surface of theoxide semiconductor film. In each of the plurality of crystal parts, thelength of a crystal arrangement part containing indium and oxygen on aplane perpendicular to the c-axes is more than 1.5 nm.

Another embodiment of the invention disclosed in this specification isan oxide semiconductor film including indium, which includes a pluralityof crystal parts. C-axes of the plurality of crystal parts are aligned.Each of the c-axes is substantially perpendicular to a surface of theoxide semiconductor film. In each of the plurality of crystal parts, thelength of a crystal arrangement part containing indium and oxygen on aplane perpendicular to the c-axes is more than or equal to 2 nm and lessthan or equal to 20 nm.

Another embodiment of the invention disclosed in this specification isan oxide semiconductor film including gallium and zinc in any of thestructures of the above-described embodiments.

Another embodiment of the invention disclosed in this specification is atransistor including the above oxide semiconductor film in its channelformation region.

Another embodiment of the invention disclosed in this specification is asemiconductor device including a circuit which includes the abovetransistor.

In the oxide semiconductor film (CAAC-OS film) disclosed in thisspecification, detachment of a metal element in the film is unlikely tooccur even when heat treatment is performed, and low photoresponse isexhibited, as compared to an amorphous oxide semiconductor film.Accordingly, the oxide semiconductor film (CAAC-OS film) disclosed inthis specification is highly stable to heat and light.

Defects typified by oxygen defects in the oxide semiconductor film whichis used for a channel formation region are preferably reduced. Defectstypified by oxygen defects function as sources for supplying carriers inthe oxide semiconductor film, which might change the electricconductivity of the oxide semiconductor film.

The number of lone electrons in the oxide semiconductor film can bemeasured as a spin density of the oxide semiconductor film by electronspin resonance (ESR), and using the spin density, the number of oxygendefects can be estimated.

The oxide semiconductor film (CAAC-OS film) disclosed in thisspecification has a low spin density. The number of oxygen defects inthe oxide semiconductor film is smaller than that in an amorphous oxidesemiconductor film.

Accordingly, a transistor including the oxide semiconductor film(CAAC-OS film) disclosed in this specification in its channel formationregion has stable electric conductivity and is more electrically stableto heat and light.

One embodiment of the present invention relates to an oxidesemiconductor film, a transistor to which the oxide semiconductor filmcan be applied, and a semiconductor device including a circuit whichincludes the transistor. For example, one embodiment of the presentinvention relates to a semiconductor device including a transistor inwhich a channel formation region is formed using an oxide semiconductoror a semiconductor device including a circuit which includes such atransistor. For example, one embodiment of the present invention relatesto an LSI, a CPU, a power device mounted in a power circuit, asemiconductor integrated circuit including a memory, a thyristor, aconverter, an image sensor, or the like, an electro-optical devicetypified by a liquid crystal display panel, a light-emitting displaydevice including a light-emitting element, or an electronic deviceincluding the aforementioned device as a component.

An oxide semiconductor film which has high stability and does not easilycause variation in electric characteristics of a transistor can beprovided.

A transistor which includes the oxide semiconductor film in its channelformation region and has stable electric characteristics can beprovided.

A highly reliable semiconductor device including the transistor can beprovided.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a TEM image of Example Sample;

FIG. 2 is a TEM image of Comparative Sample;

FIG. 3 illustrates one embodiment of a structure of a crystal part;

FIG. 4A is a graph showing XRD results of Example Sample, and FIG. 4B isa graph showing XRD results of Comparative Sample;

FIGS. 5A to 5E are each a cross-sectional view of an embodiment of asemiconductor device;

FIGS. 6A to 6C are views each illustrating an electronic appliance;

FIGS. 7A to 7C are views illustrating an electronic appliance; and

FIGS. 8A to 8C are views illustrating electronic appliances.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments and an example of the invention disclosed inthis specification will be described in detail with reference to theaccompanying drawings. Note that the invention disclosed in thisspecification is not limited to the following description, and it iseasily understood by those skilled in the art that modes and details canbe variously changed without departing from the spirit and the scope ofthe invention. Therefore, the invention disclosed in this specificationis not construed as being limited to the description of the followingembodiments and example. Note that the ordinal numbers such as “first”and “second” in this specification are used for convenience and do notdenote the order of steps and the stacking order of layers. In addition,the ordinal numbers in this specification do not denote particular nameswhich specify the present invention.

Embodiment 1

An oxide semiconductor film of the invention disclosed in thisspecification is an oxide semiconductor (CAAC-OS) film which includes acrystal part whose c-axis is substantially perpendicular to a surface ofthe oxide semiconductor film and in which the length of a crystalarrangement part containing indium and oxygen on a plane perpendicularto the c-axis in the crystal part is more than 1.5 nm (preferably morethan or equal to 2 nm and less than or equal to 20 nm).

An oxide semiconductor film of the invention disclosed in thisspecification is a CAAC-OS film including a crystal part in which acrystal arrangement part containing indium and oxygen on a planeperpendicular to the c-axis and having a length more than at least 1.5nm (preferably more than or equal to 2 nm and less than or equal to 20nm) is observed.

An oxide semiconductor film may be in a non-single-crystal state, forexample. The non-single-crystal state is, for example, structured by atleast one of c-axis aligned crystal (CAAC), polycrystal, microcrystal,and an amorphous part. The density of defect states of an amorphous partis higher than those of microcrystal and CAAC. The density of defectstates of microcrystal is higher than that of CAAC. Note that an oxidesemiconductor including CAAC is referred to as a CAAC-OS (c-axis alignedcrystalline oxide semiconductor).

For example, an oxide semiconductor film may include a CAAC-OS. In theCAAC-OS, for example, c-axes are aligned, and a-axes and/or b-axes arenot macroscopically aligned.

For example, an oxide semiconductor film may include microcrystal. Notethat an oxide semiconductor including microcrystal is referred to as amicrocrystalline oxide semiconductor. A microcrystalline oxidesemiconductor film includes microcrystal (also referred to asnanocrystal) with a size greater than or equal to 1 nm and less than 10nm, for example.

For example, an oxide semiconductor film may include an amorphous part.Note that an oxide semiconductor including an amorphous part is referredto as an amorphous oxide semiconductor. An amorphous oxide semiconductorfilm, for example, has disordered atomic arrangement and no crystallinecomponent. Alternatively, an amorphous oxide semiconductor film is, forexample, absolutely amorphous and has no crystal part.

Note that an oxide semiconductor film may be a mixed film including anyof a CAAC-OS, a microcrystalline oxide semiconductor, and an amorphousoxide semiconductor. The mixed film, for example, includes a region ofan amorphous oxide semiconductor, a region of a microcrystalline oxidesemiconductor, and a region of a CAAC-OS. Further, the mixed film mayhave a stacked structure including a region of an amorphous oxidesemiconductor, a region of a microcrystalline oxide semiconductor, and aregion of a CAAC-OS, for example.

Note that an oxide semiconductor film may be in a single-crystal state,for example.

An oxide semiconductor film preferably includes a plurality of crystalparts. In each of the crystal parts, a c-axis is preferably aligned in adirection parallel to a normal vector of a surface where the oxidesemiconductor film is formed or a normal vector of a surface of theoxide semiconductor film. Note that, among crystal parts, the directionsof the a-axis and the b-axis of one crystal part may be different fromthose of another crystal part. An example of such an oxide semiconductorfilm is a CAAC-OS film.

The CAAC-OS film, for example, includes an oxide semiconductor withcrystal parts. Note that in most cases, a crystal part in the CAAC-OSfilm fits inside a cube whose one side is less than 100 nm. In an imageobtained with a transmission electron microscope (TEM), a grain boundaryin the CAAC-OS film is not clearly found. Thus, in the CAAC-OS film, areduction in electron mobility due to the grain boundary is suppressed.

In each of the crystal parts included in the CAAC-OS film, for example,a c-axis is aligned in a direction parallel to a normal vector of asurface where the CAAC-OS film is formed or a normal vector of a surfaceof the CAAC-OS film. Further, in each of the crystal parts, metal atomsare arranged in a triangular or hexagonal configuration when seen fromthe direction perpendicular to the a-b plane, and metal atoms arearranged in a layered manner or metal atoms and oxygen atoms arearranged in a layered manner when seen from the direction perpendicularto the c-axis. Note that, among the crystal parts, the directions of thea-axis and the b-axis of one crystal part are different from those ofanother crystal part. In other words, because the a-axis and the b-axisvary among the crystal parts in the CAAC-OS film although the c-axes arealigned, the CAAC-OS film is not an epitaxially grown film. In thisspecification, a term “perpendicular” includes a range from 80° to 100°,preferably from 85° to 95°. In addition, a term “parallel” includes arange from −10° to 10°, preferably from −5° to 5°.

In the CAAC-OS film, distribution of crystal parts is not necessarilyuniform. For example, in the formation process of the CAAC-OS film, inthe case where crystal growth occurs from a surface side of the oxidesemiconductor film, the proportion of crystal parts in the vicinity ofthe surface of the oxide semiconductor film is higher than that in thevicinity of the surface where the oxide semiconductor film is formed insome cases.

Since the c-axes of the crystal parts included in the CAAC-OS film arealigned in the direction parallel to a normal vector of a surface wherethe CAAC-OS film is formed or a normal vector of a surface of theCAAC-OS film, the directions of the c-axes may be different from eachother depending on the shape of the CAAC-OS film (the cross-sectionalshape of the surface where the CAAC-OS film is formed or thecross-sectional shape of the surface of the CAAC-OS film). Note that thefilm deposition is accompanied with the formation of the crystal partsor followed by the formation of the crystal parts throughcrystallization treatment such as heat treatment. Hence, the c-axes ofthe crystal parts are aligned in the direction parallel to a normalvector of the surface where the CAAC-OS film is formed or a normalvector of the surface of the CAAC-OS film.

In a transistor using the CAAC-OS film, change in electriccharacteristics due to irradiation with visible light or ultravioletlight is small. Thus, the transistor has high reliability.

An example of a crystal structure of the crystal part in the CAAC-OSfilm is described with reference to FIG. 3. In FIG. 3, the upwarddirection corresponds to the c-axis direction and a plane perpendicularto the c-axis direction corresponds to the a-b plane, unless otherwisespecified. In this embodiment, a structure of an In—Ga—Zn—O-basedcrystal which is included in a crystal part of an oxide semiconductorfilm containing indium, gallium, and zinc is exemplified. In FIG. 3, agray sphere represents an indium atom, a white sphere represents agallium atom or a zinc atom, a black sphere represents an oxygen atom,and an arrow represents the c-axis direction of the In—Ga—Zn—O-basedcrystal.

As illustrated in FIG. 3, in the structure of the c-axis alignedIn—Ga—Zn—O-based crystal, indium and oxygen (In—O) has such crystalarrangement that indium atoms and oxygen atoms are arranged in adirection perpendicular to the c-axis. The oxide semiconductor film ofthe invention disclosed in this specification which contains indium,gallium, and zinc is a film including a crystal part in which the lengthof a crystal arrangement part containing indium and oxygen is more thanat least 1.5 nm (preferably more than or equal to 2 nm and less than orequal to 20 nm).

An oxide semiconductor used for the oxide semiconductor film of theinvention disclosed in this specification contains at least indium (In).In particular, the oxide semiconductor preferably contains In and zinc(Zn). As a stabilizer for reducing variation in electric characteristicsof a transistor including the oxide semiconductor film, the oxidesemiconductor preferably contains gallium (Ga), tin (Sn), hafnium (Hf),aluminum (Al), and/or zirconium (Zr) in addition to In and Zn.

As another stabilizer, one or plural kinds of lanthanoid such aslanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium(Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy),holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium(Lu) may be contained.

As the oxide semiconductor, for example, indium oxide; a two-componentmetal oxide such as an In—Zn-based oxide, an In—Mg-based oxide, or anIn—Ga-based oxide; a three-component metal oxide such as anIn—Ga—Zn-based oxide (also referred to as IGZO), an In—Al—Zn-basedoxide, an In—Sn—Zn-based oxide, an In—Hf—Zn-based oxide, anIn—La—Zn-based oxide, an In—Ce—Zn-based oxide, an In—Pr—Zn-based oxide,an In—Nd—Zn-based oxide, an In—Sm—Zn-based oxide, an In—Eu—Zn-basedoxide, an In—Gd—Zn-based oxide, an In—Tb—Zn-based oxide, anIn—Dy—Zn-based oxide, an In—Ho—Zn-based oxide, an In—Er—Zn-based oxide,an In—Tm—Zn-based oxide, an In—Yb—Zn-based oxide, or an In—Lu—Zn-basedoxide; or a four-component metal oxide such as an In—Sn—Ga—Zn-basedoxide, an In—Hf—Ga—Zn-based oxide, an In—Al—Ga—Zn-based oxide, anIn—Sn—Al—Zn-based oxide, an In—Sn—Hf—Zn-based oxide, or anIn—Hf—Al—Zn-based oxide can be used.

Note that here, for example, an In—Ga—Zn-based oxide means an oxidecontaining In, Ga, and Zn as main components, and there is no limitationon the ratio of In:Ga:Zn. The In—Ga—Zn-based oxide may contain a metalelement other than the In, Ga, and Zn.

Alternatively, a material represented by InMO₃(ZnO)_(m) (m>0 issatisfied, and m is not an integer) may be used as an oxidesemiconductor. Note that M represents one or more metal elementsselected from Ga, Fe, Mn, and Co. Alternatively, as the oxidesemiconductor, a material represented by In₂SnO₅(ZnO)_(n) (n>0 issatisfied, and n is an integer) may be used.

For example, an In—Ga—Zn-based oxide with an atomic ratio ofIn:Ga:Zn=1:1:1 (=⅓:⅓:⅓), In:Ga:Zn=2:2:1 (=⅖:⅖:⅕), In:Ga:Zn=3:1:2(=½:⅙:⅓), or any of oxides whose composition is in the neighborhood ofthe above compositions can be used. Alternatively, an In—Sn—Zn-basedoxide with an atomic ratio of In:Sn:Zn=1:1:1 (=⅓:⅓:⅓), In:Sn:Zn=2:1:3(=⅓:⅙:½), or In:Sn:Zn=2:1:5 (=¼:⅛:⅝), or any of oxides whose compositionis in the neighborhood of the above compositions may be used.

However, without limitation to the materials given above, a materialwith an appropriate composition may be used as the oxide semiconductorcontaining indium depending on needed semiconductor characteristics(e.g., mobility, threshold voltage, and variation). In order to obtainthe needed semiconductor characteristics, it is preferable that thecarrier concentration, the impurity concentration, the defect density,the atomic ratio between a metal element and oxygen, the interatomicdistance, the density, and the like be set to appropriate values.

Note that for example, the expression “the composition of an oxidecontaining In, Ga, and Zn at the atomic ratio, In:Ga:Zn=a:b:c (a+b+c=1),is in the neighborhood of the composition of an oxide containing In, Ga,and Zn at the atomic ratio, In:Ga:Zn=A:B:C (A+B+C=1)” means that a, b,and c satisfy the following relation: (a−A)²+(b−B)²+(c−C)²≦r². Forexample, r may be 0.05. The same applies to other oxides.

Note that part of oxygen included in the oxide semiconductor film may besubstituted with nitrogen.

An oxide semiconductor film of the invention disclosed in thisspecification can be formed by a sputtering method, a molecular beamepitaxy (MBE) method, a CVD method, a pulse laser deposition method, anatomic layer deposition (ALD) method, or the like. Alternatively, theoxide semiconductor film may be formed with a sputtering apparatus wheredeposition is performed with surfaces of a plurality of substrates setsubstantially perpendicular to a surface of a sputtering target. Theoxide semiconductor film can be formed, for example, by any of thefollowing methods: a method for forming a crystal part which isc-axis-aligned in a direction substantially perpendicular to a surfaceof the oxide semiconductor film by performing deposition while heattreatment is performed; a method for forming a crystal part which isc-axis-aligned in a direction substantially perpendicular to the surfaceof the oxide semiconductor film by crystallization treatment such asheat treatment after the deposition; and a method for forming an oxidesemiconductor film including a crystal part which is c-axis-aligned in adirection substantially perpendicular to the surface of the oxidesemiconductor film by depositing a film over a CAAC-OS film.

For example, the CAAC-OS film is formed by a sputtering method with apolycrystalline oxide semiconductor sputtering target. When ions collidewith the sputtering target, a crystal region included in the sputteringtarget may be separated from the target along an a-b plane; in otherwords, a sputtered particle having a plane parallel to an a-b plane(flat-plate-like sputtered particle or pellet-like sputtered particle)may flake off from the sputtering target. In that case, theflat-plate-like sputtered particle reaches a substrate while maintainingits crystal state, whereby the CAAC-OS film can be formed.

For the deposition of the CAAC-OS film, the following conditions arepreferably used.

By reducing the amount of impurities entering the CAAC-OS film duringthe deposition, the crystal state can be prevented from being broken bythe impurities. For example, the concentration of impurities (e.g.,hydrogen, water, carbon dioxide, or nitrogen) which exist in adeposition chamber may be reduced. Furthermore, the concentration ofimpurities in a deposition gas may be reduced. Specifically, adeposition gas whose dew point is −80° C. or lower, preferably −100° C.or lower is used.

By increasing the substrate heating temperature during the deposition,migration of a sputtered particle is likely to occur after the sputteredparticle reaches a substrate surface. Specifically, the substrateheating temperature during the deposition is higher than or equal to100° C. and lower than or equal to 740° C., preferably higher than orequal to 200° C. and lower than or equal to 500° C. By increasing thesubstrate heating temperature during the deposition, when theflat-plate-like sputtered particle reaches the substrate, migrationoccurs on the substrate surface, so that a flat plane of theflat-plate-like sputtered particle is attached to the substrate.

Furthermore, it is preferable that the proportion of oxygen in thedeposition gas be increased and the power be optimized in order toreduce plasma damage at the deposition. The proportion of oxygen in thedeposition gas is 30 vol % or higher, preferably 100 vol %.

As an example of the sputtering target, an In—Ga—Zn—O compound target isdescribed below.

The In—Ga—Zn—O compound target, which is polycrystalline, is made bymixing InO_(X) powder, GaO_(Y) powder, and ZnO_(Z) powder in apredetermined molar ratio, applying pressure, and performing heattreatment at a temperature higher than or equal to 1000° C. and lowerthan or equal to 1500° C. Note that X, Y, and Z are each a givenpositive number. Here, the predetermined molar ratio of InO_(X) powderto GaO_(Y) powder and ZnO_(Z) powder is, for example, 2:2:1, 8:4:3,3:1:1, 1:1:1, 4:2:3, or 3:1:2. The kinds of powder and the molar ratiofor mixing powder may be determined as appropriate depending on thedesired sputtering target.

Further, the oxide semiconductor film including a crystal part of theinvention disclosed in this specification can achieve higher mobility byimprovement in flatness of the surface. In order to improve the surfaceflatness, the oxide semiconductor film is preferably formed on a flatsurface. Specifically, the oxide semiconductor film is preferably formedon a surface with an average surface roughness (Ra) of less than orequal to 1 nm, preferably less than or equal to 0.3 nm, more preferablyless than or equal to 0.1 nm.

Note that, R_(a) is obtained by three-dimension expansion of arithmeticaverage roughness that is defined by JIS B 0601:2001 (ISO4287:1997) soas to be applied to a curved plane. The R_(a) can be expressed as an“average value of the absolute values of deviations from a referencesurface to a specific surface” and is defined by the formula below.

$\begin{matrix}{{Ra} = {\frac{1}{S_{0}}{\int_{y_{1}}^{y_{2}}{\int_{x_{1}}^{x_{2}}{{{{f\left( {x,y} \right)} - Z_{0}}}{x}{y}}}}}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack\end{matrix}$

Here, the specific surface is a surface which is a target of roughnessmeasurement, and is a quadrilateral region which is specified by fourpoints represented by the coordinates (x₁, y₁, f(x₁, y₁)), (x₁, y₂,f(x₁, y₂)), (x₂, y₁, f(x₂, y₁)), and (x₂, y₂, f(x₂, y₂)). So representsthe area of a rectangle which is obtained by projecting the specificsurface on the xy plane, and Z₀ represents the height of the referencesurface (the average height of the specific surface). Ra can be measuredusing an atomic force microscope (AFM).

In the oxide semiconductor film (CAAC-OS film) disclosed in thisspecification, detachment of a metal element in the film is unlikely tooccur even when heat treatment is performed, and low photoresponse isexhibited, as compared to an amorphous oxide semiconductor film.Accordingly, the oxide semiconductor film (CAAC-OS film) disclosed inthis specification is highly stable to heat and light.

Further, the oxide semiconductor film (CAAC-OS film) disclosed in thisspecification has a low spin density. The number of oxygen defects inthe oxide semiconductor film is smaller than that in an amorphous oxidesemiconductor film.

Accordingly, a transistor including the oxide semiconductor film(CAAC-OS film) disclosed in this specification in its channel formationregion has stable electric conductivity and is more electrically stableto heat and light.

Thus, a highly reliable semiconductor device which includes a transistorincluding the oxide semiconductor film in its channel formation regioncan be provided.

The structures, methods, and the like described in this embodiment canbe combined as appropriate with any of the structures, methods, and thelike described in the other embodiments.

Embodiment 2

A transistor included in a semiconductor device of the inventiondisclosed in this specification includes, in its channel formationregion, the CAAC-OS film described in Embodiment 1 which includes acrystal part whose c-axis is substantially perpendicular to a surface ofthe CAAC-OS film and in which the length of a crystal arrangement partcontaining indium and oxygen on a plane perpendicular to the c-axis ismore than 1.5 nm (preferably, more than or equal to 2 nm and less thanor equal to 20 nm).

There is no particular limitation on the structure of the transistorincluded in the semiconductor device of the invention disclosed in thisspecification; for example, a staggered type or a planar type having atop-gate structure or a bottom-gate structure can be employed. Further,the transistor may have a single gate structure including one channelformation region, or a multi gate structure such as a double gatestructure including two channel formation regions or a triple gatestructure including three channel formation regions. Alternatively, thetransistor may have a dual-gate structure including two gate electrodelayers positioned above and below a channel formation region with a gateinsulating film provided therebetween.

Examples of transistors each included in the semiconductor device of theinvention disclosed in this specification are illustrated in FIGS. 5A to5E. The transistors illustrated in FIGS. 5A to 5E each use, as an oxidesemiconductor film 403, the CAAC-OS film described in Embodiment 1 whichincludes a crystal part whose c-axis is substantially perpendicular tothe surface of the CAAC-OS film and in which the length of a crystalarrangement part containing indium and oxygen on a plane perpendicularto the c-axis is more than 1.5 nm (preferably, more than or equal to 2nm and less than or equal to 20 nm).

A transistor 410 illustrated in FIG. 5A is a kind of bottom-gatetransistor and is also called an inverted staggered transistor.

The transistor 410 includes, over a substrate 400 having an insulatingsurface, a gate electrode layer 401, a gate insulating film 402, theoxide semiconductor film 403, a source electrode layer 405 a, and adrain electrode layer 405 b. In addition, an insulating film 407 whichcovers the transistor 410 and is stacked over the oxide semiconductorfilm 403 is provided.

A transistor 420 illustrated in FIG. 5B is a kind of bottom-gatetransistor referred to as a channel-protective transistor (also referredto as a channel-stop transistor) and is also referred to as aninverted-staggered transistor.

The transistor 420 includes, over the substrate 400 having an insulatingsurface, the gate electrode layer 401, the gate insulating film 402, theoxide semiconductor film 403, an insulating film 427 functioning as achannel protective layer covering a channel formation region of theoxide semiconductor film 403, the source electrode layer 405 a, and thedrain electrode layer 405 b. The insulating film 407 is provided tocover the transistor 420.

A transistor 430 shown in FIG. 5C is a bottom-gate transistor andincludes, over the substrate 400 having an insulating surface, the gateelectrode layer 401, the gate insulating film 402, the source electrodelayer 405 a, the drain electrode layer 405 b, and the oxidesemiconductor film 403. The insulating film 407 which covers thetransistor 430 and is in contact with the oxide semiconductor film 403is provided.

In the transistor 430, the gate insulating film 402 is provided over andin contact with the substrate 400 and the gate electrode layer 401, andthe source electrode layer 405 a and the drain electrode layer 405 b areprovided over and in contact with the gate insulating film 402. Further,the oxide semiconductor film 403 is provided over the gate insulatingfilm 402, the source electrode layer 405 a, and the drain electrodelayer 405 b.

A transistor 440 illustrated in FIG. 5D is one kind of top-gatetransistor. The transistor 440 includes, over the substrate 400 havingan insulating surface, an insulating film 437, the oxide semiconductorfilm 403, the source electrode layer 405 a, the drain electrode layer405 b, the gate insulating film 402, and the gate electrode layer 401.The source electrode layer 405 a and the drain electrode layer 405 b areelectrically connected to the oxide semiconductor film 403 in an openingformed in the gate insulating film 402, the insulating film 407, and theinsulating film 415.

A transistor 450 illustrated in FIG. 5E is one kind of top-gatetransistor. The transistor 450 includes, over the substrate 400 havingan insulating surface, the insulating film 437, the source electrodelayer 405 a, the drain electrode layer 405 b, the oxide semiconductorfilm 403, the gate insulating film 402, and the gate electrode layer401.

There is no particular limitation on a substrate that can be used as thesubstrate 400 having an insulating surface as long as it has heatresistance enough to withstand heat treatment performed later. A varietyof glass substrates for electronics industry, such as a bariumborosilicate glass substrate and an aluminoborosilicate glass substrate,can be used as the substrate 400. Note that as the substrate, asubstrate having a thermal expansion coefficient of greater than orequal to 25×10⁻⁷/° C. and less than or equal to 50×10⁻⁷/° C. (preferablygreater than or equal to 30×10⁻⁷/° C. and less than or equal to40×10⁻⁷/° C.) and a strain point of higher than or equal to 650° C. andlower than or equal to 750° C. (preferably higher than or equal to 700°C. and lower than or equal to 740° C.) is preferably used.

In the case where a large-sized glass substrate with any of the 5thgeneration (1000 mm×1200 mm or 1300 mm×1500 mm), the 6th generation(1500 mm×1800 mm), the 7th generation (1870 mm×2200 mm), the 8thgeneration (2200 mm×2500 mm), the 9th generation (2400 mm×2800 mm), andthe 10th generation (2880 mm×3130 mm) is used, minute processing mightbecome difficult owing to shrinkage of the substrate caused by heattreatment or the like in the process for manufacturing a semiconductordevice. Therefore, when such a large-sized glass substrate is used asthe substrate, the one with a small shrinkage is preferably used. Forexample, as the substrate, a large-sized glass substrate whose shrinkageby heat treatment for one hour at preferably 450° C., more preferably500° C. is less than or equal to 20 ppm, preferably less than or equalto 10 ppm, more preferably less than or equal to 5 ppm may be used.

Alternatively, a ceramic substrate, a quartz substrate, a sapphiresubstrate, or the like can be used as the substrate 400. Alternatively,a single crystal semiconductor substrate or a polycrystallinesemiconductor substrate made of silicon or silicon carbide, a compoundsemiconductor substrate made of silicon germanium or the like, an SOIsubstrate, or the like can be used. Alternatively, any of thesesubstrates over which a semiconductor element is provided may be used.

The semiconductor device may be manufactured using a flexible substrateas the substrate 400. To manufacture a flexible semiconductor device,the transistor 440 including the oxide semiconductor film 403 may bedirectly formed over a flexible substrate; or alternatively, thetransistor 440 including the oxide semiconductor film 403 may be formedover a manufacturing substrate, and then the transistor may be separatedfrom the manufacturing substrate and transferred to a flexiblesubstrate. Note that, in order to separate the transistor from themanufacturing substrate and transfer it to the flexible substrate, aseparation layer may be provided between the manufacturing substrate andthe transistor 440 including the oxide semiconductor film.

The insulating film 437 can be formed by a plasma CVD method, asputtering method, or the like using an oxide insulating film of siliconoxide, silicon oxynitride, aluminum oxide, aluminum oxynitride, hafniumoxide, gallium oxide, or the like; a nitride insulating film of siliconnitride, silicon nitride oxide, aluminum nitride, aluminum nitrideoxide, or the like; or a mixed material thereof.

The gate electrode layer 401 can be formed with the use of a metalmaterial such as molybdenum, titanium, tantalum, tungsten, aluminum,copper, chromium, neodymium, or scandium or an alloy material whichcontains any of these materials as its main component. A semiconductorfilm which is doped with an impurity element such as phosphorus and istypified by a polycrystalline silicon film, or a silicide film of nickelsilicide or the like can also be used as the gate electrode layer 401.The gate electrode layer 401 has either a single-layer structure or astacked-layer structure.

The gate electrode layer 401 can also be formed using a conductivematerial such as indium oxide-tin oxide, indium oxide containingtungsten oxide, indium zinc oxide containing tungsten oxide, indiumoxide containing titanium oxide, indium tin oxide containing titaniumoxide, indium oxide-zinc oxide, or indium tin oxide to which siliconoxide is added. The gate electrode layer 401 can have a stackedstructure of the above conductive material and the above metal material.

As the gate electrode layer 401, a metal oxide film containing nitrogen,specifically, an In—Ga—Zn—O film containing nitrogen, an In—Sn—O filmcontaining nitrogen, an In—Ga—O film containing nitrogen, an In—Zn—Ofilm containing nitrogen, a Sn—O film containing nitrogen, an In—O filmcontaining nitrogen, or a metal nitride (e.g., InN or SnN) film can beused.

The gate insulating film 402 can be formed by a sputtering method or aCVD method using a deposition gas. As the CVD method, an LPCVD method, aplasma CVD method, or the like can be used, and as another method, acoating film or the like can also be used.

The gate insulating film 402 can be formed using a silicon oxide film, agallium oxide film, an aluminum oxide film, a silicon nitride film, asilicon oxynitride film, an aluminum oxynitride film, or a siliconnitride oxide film.

When the gate insulating film 402 is formed using a high-k material suchas hafnium oxide, yttrium oxide, hafnium silicate (HfSi_(x)O_(y) (x>0,y>0)), hafnium silicate to which nitrogen is added (HfSiO_(x)N_(y) (x>0,y>0)), hafnium aluminate (HfAl_(x)O_(y) (x>0, y>0)), or lanthanum oxide,gate leakage current can be reduced. Further, the gate insulating film402 has either a single-layer structure or a stacked-layer structure.

As the source electrode layer 405 a and the drain electrode layer 405 b,for example, a metal film containing an element selected from Al, Cr,Cu, Ta, Ti, Mo, and W, or a metal nitride film containing any of theabove elements as a component (e.g., a titanium nitride film, amolybdenum nitride film, or a tungsten nitride film) can be used. Ametal film having a high melting point such as Ti, Mo, W, or the like ora metal nitride film of any of these elements (a titanium nitride film,a molybdenum nitride film, and a tungsten nitride film) may be stackedon one of or both of a lower side or an upper side of a metal film ofAl, Cu, or the like. Alternatively, the conductive film used as thesource electrode layer 405 a and the drain electrode layer 405 b may beformed using conductive metal oxide. As the conductive metal oxide,indium oxide (In₂O₃), tin oxide (SnO₂), zinc oxide (ZnO), indiumoxide-tin oxide (In₂O₃—SnO₂), indium oxide-zinc oxide (In₂O₃—ZnO), orany of these metal oxide materials in which silicon oxide is containedcan be used.

The insulating films 407, 415, and 427 can be formed by a plasma CVDmethod, a sputtering method, an evaporation method, or the like.

Each of the insulating films 407, 415, and 427 can be a single layer ora stacked layer of a silicon oxide film, a silicon oxynitride film, analuminum oxide film, an aluminum oxynitride film, an inorganicinsulating film such as a gallium oxide film, a hafnium oxide film, amagnesium oxide film, a zirconium oxide film, a lanthanum oxide film, abarium oxide film, a silicon nitride film, a silicon nitride oxide film,an aluminum nitride film, or an aluminum nitride oxide film, or thelike.

In addition, a planarization insulating film may be formed over thetransistor in order that surface unevenness due to the transistor isreduced. As the planarization insulating film, an organic material suchas a polyimide-, acrylic-, or benzocyclobutene-based resin can be used.Other than such organic materials, it is also possible to use alow-dielectric constant material (a low-k material) or the like. Notethat the planarization insulating film may be formed by stacking aplurality of insulating films formed from these materials.

In the oxide semiconductor film (the CAAC-OS film) 403 disclosed in thisspecification, detachment of a metal element in the film is unlikely tooccur even when heat treatment is performed, and low photoresponse isexhibited, as compared to an amorphous oxide semiconductor film.Accordingly, the oxide semiconductor film (CAAC-OS film) disclosed inthis specification is highly stable to heat and light.

Further, the oxide semiconductor film (CAAC-OS film) 403 disclosed inthis specification has a low spin density. The number of oxygen defectsin the oxide semiconductor film is smaller than that in an amorphousoxide semiconductor film

Therefore, the transistors 410, 420, 430, 440, and 450 each including,in its channel formation region, the oxide semiconductor film (theCAAC-OS film) disclosed in this specification have stable electricconductivity and are more electrically stable to heat and light.

Accordingly, a highly reliable semiconductor device which includes atransistor including the oxide semiconductor film in its channelformation region can be provided.

The structures, methods, and the like described in this embodiment canbe combined as appropriate with any of the structures, methods, and thelike described in the other embodiments.

Embodiment 3

The transistor of one embodiment of the invention disclosed in thisspecification can be applied to semiconductor devices having a varietyof functions. For example, the transistor can be applied to a memorydevice, a central processing unit (CPU), or an LSI such as a digitalsignal processor (DSP), a custom LSI, or a field programmable gate array(FPGA).

A semiconductor device disclosed in this specification can be applied toa variety of electronic appliances (including game machines). Examplesof the electronic appliances include display devices of televisions,monitors, and the like, lighting devices, desktop personal computers andlaptop personal computers, word processors, image reproduction deviceswhich reproduce still images or moving images stored in recording mediasuch as digital versatile discs (DVDs), portable compact disc (CD)players, radio receivers, tape recorders, headphone stereos, stereos,cordless phone handsets, transceivers, portable wireless devices, mobilephones, car phones, portable game machines, calculators, portableinformation terminals, electronic notebooks, e-book readers, electronictranslators, audio input devices, cameras such as digital still camerasand video cameras, electric shavers, high-frequency heating appliancessuch as microwave ovens, electric rice cookers, electric washingmachines, electric vacuum cleaners, air-conditioning systems such as airconditioners, dishwashers, dish dryers, clothes dryers, futon dryers,electric refrigerators, electric freezers, electricrefrigerator-freezers, freezers for preserving DNA, smoke detectors,radiation counters, and medical equipment such as dialyzers. Further,the examples include industrial equipment such as guide lights, trafficlights, belt conveyors, elevators, escalators, industrial robots, andpower storage systems. In addition, oil engines, moving objects drivenby electric motors using power from the non-aqueous secondary batteries,and the like are also included in the category of electronic appliances.Examples of the moving objects include electric vehicles (EV), hybridelectric vehicles (HEV) which include both an internal-combustion engineand a motor, plug-in hybrid electric vehicles (PHEV), tracked vehiclesin which caterpillar tracks are substituted for wheels of thesevehicles, motorized bicycles including motor-assisted bicycles,motorcycles, electric wheelchairs, golf carts, boats or ships,submarines, helicopters, aircrafts, rockets, artificial satellites,space probes, planetary probes, spacecrafts, and the like. Specificexamples of these electronic appliances are illustrated in FIGS. 6A to6C.

FIG. 6A illustrates a table 9000 having a display portion. In the table9000, a display portion 9003 is incorporated in a housing 9001 and animage can be displayed on the display portion 9003. Note that thehousing 9001 is supported by four leg portions 9002. Further, a powercord 9005 for supplying power is provided for the housing 9001.

The transistor of one embodiment of the invention disclosed in thisspecification can be used for the display portion 9003, so that theelectronic appliance can have high reliability.

The display portion 9003 has a touch-input function. When a user touchesdisplayed buttons 9004 which are displayed on the display portion 9003of the table 9000 with his/her finger or the like, the user can carryout operation of the screen and input of information. Further, when thetable may be made to communicate with home appliances or control thehome appliances, the table 9000 may function as a control device whichcontrols the home appliances by operation on the screen. For example,with the use of a semiconductor device having an image sensor function,the display portion 9003 can have a touch-input function.

Further, the screen of the display portion 9003 can be placedperpendicular to a floor with a hinge provided for the housing 9001;thus, the table 9000 can also be used as a television device. When atelevision device having a large screen is set in a small room, an openspace is reduced; however, when a display portion is incorporated in atable, a space in the room can be efficiently used.

FIG. 6B illustrates a portable music player, which includes, in a mainbody 3021, a display portion 3023, a fixing portion 3022 with which themain body is worn on the ear, a speaker, an operation button 3024, anexternal memory slot 3025, and the like. By application of thetransistor of one embodiment of the invention disclosed in thisspecification or a memory including the transistor to a memory, a CPU,or the like which is incorporated in the main body 3021, a portablemusic player (PDA) whose power consumption is further reduced can beachieved.

Furthermore, when the portable music player illustrated in FIG. 6B hasan antenna, a microphone function, or a wireless communication functionand is used with a mobile phone, a user can talk on the phone wirelesslyin a hands-free way while driving a car or the like.

FIG. 6C illustrates a computer, which includes a main body 9201including a CPU, a housing 9202, a display portion 9203, a keyboard9204, an external connection port 9205, a pointing device 9206, and thelike. The computer includes a semiconductor device manufacturedaccording to one embodiment of the present invention for the displayportion 9203. When a CPU including the transistor of one embodiment ofthe invention disclosed in this specification is used, power consumptionof the computer can be reduced.

FIGS. 7A and 7B illustrate a tablet terminal that can be folded. In FIG.7A, the tablet terminal is opened, and includes a housing 9630, adisplay portion 9631 a, a display portion 9631 b, a display-modeswitching button 9034, a power button 9035, a power-saving-modeswitching button 9036, a clip 9033, and an operation button 9638.

In such a portable device illustrated in FIGS. 7A and 7B, an SRAM or aDRAM is used as a memory for temporarily storing image data. Forexample, a semiconductor device including the transistor of oneembodiment of the invention disclosed in this specification can be usedas a memory. The semiconductor device employed for the memory enableshigh-speed writing and reading of data, long-time retention of data, andsufficient reduction of power consumption.

A touch panel area 9632 a can be provided in a part of the displayportion 9631 a, in which data can be input by touching displayedoperation keys 9638. Although half of the display portion 9631 a hasonly a display function and the other half has a touch panel function,one embodiment of the present invention is not limited to the structure.The whole display portion 9631 a may have a touch panel function. Forexample, the display portion 9631 a can display a keyboard in the wholeregion to be used as a touch panel, and the display portion 9631 b canbe used as a display screen.

Like the display portion 9631 a, part of the display portion 9631 b canbe a touch panel region 9632 b. When a finger, a stylus, or the liketouches the place where a button 9639 for switching to keyboard displayis displayed in the touch panel, keyboard buttons can be displayed onthe display portion 9631 b.

Touch input can be performed concurrently on the touch panel regions9632 a and 9632 b.

The display-mode switching button 9034 allows switching between alandscape mode and a portrait mode, color display and black-and-whitedisplay, and the like. With the power-saving-mode switching button 9036,the luminance of display can be optimized in accordance with the amountof external light at the time when the tablet is in use, which isdetected with an optical sensor incorporated in the tablet. The tabletmay include another detection device such as a sensor for detectingorientation (e.g., a gyroscope or an acceleration sensor) in addition tothe optical sensor.

Although the display portion 9631 a and the display portion 9631 b havethe same display area in FIG. 7A, an embodiment of the present inventionis not limited to this example. The display portion 9631 a and thedisplay portion 9631 b may have different areas or different displayquality. For example, one of them may be a display panel that candisplay higher-definition images than the other.

FIG. 7B illustrates the tablet terminal folded, which includes thehousing 9630, a solar battery 9633, a charge and discharge controlcircuit 9634, a battery 9635, and a DCDC converter 9636. Note that FIG.7B shows an example in which the charge and discharge control circuit9634 includes the battery 9635 and the DCDC converter 9636.

Since the tablet terminal can be folded in two, the housing 9630 can beclosed when the tablet terminal is not in use. Thus, the displayportions 9631 a and 9631 b can be protected, thereby providing a tabletterminal with high endurance and high reliability for long-term use.

The tablet terminal illustrated in FIGS. 7A and 7B can have otherfunctions such as a function of displaying various kinds of data (e.g.,a still image, a moving image, and a text image), a function ofdisplaying a calendar, a date, the time, or the like on the displayportion, a touch-input function of operating or editing the datadisplayed on the display portion by touch input, and a function ofcontrolling processing by various kinds of software (programs).

The solar battery 9633, which is attached on the surface of the tabletterminal, supplies electric power to a touch panel, a display portion,an image signal processor, and the like. Note that the solar battery9633 can be provided on one or both surfaces of the housing 9630, sothat the battery 9635 can be charged efficiently, which is preferable.When a lithium ion battery is used as the battery 9635, there is anadvantage of downsizing or the like.

The structure and operation of the charge and discharge control circuit9634 illustrated in FIG. 7B are described with reference to a blockdiagram of FIG. 7C. FIG. 7C illustrates the solar battery 9633, thebattery 9635, the DCDC converter 9636, a converter 9637, switches SW1 toSW3, and the display portion 9631. The battery 9635, the DCDC converter9636, the converter 9637, and the switches SW1 to SW3 correspond to thecharge and discharge control circuit 9634 in FIG. 7B.

First, an example of operation in the case where power is generated bythe solar battery 9633 using external light is described. The voltage ofpower generated by the solar battery 9633 is raised or lowered by theDCDC converter 9636 so that a voltage for charging the battery 9635 isobtained. When the display portion 9631 is operated with the power fromthe solar battery 9633, the switch SW1 is turned on and the voltage ofthe power is raised or lowered by the converter 9637 to a voltage neededfor operating the display portion 9631. In addition, when display on thedisplay portion 9631 is not performed, the switch SW1 is turned off anda switch SW2 is turned on so that charge of the battery 9635 may beperformed.

Here, the solar battery 9633 is shown as an example of a powergeneration means; however, there is no particular limitation on a way ofcharging the battery 9635, and the battery 9635 may be charged withanother power generation means such as a piezoelectric element or athermoelectric conversion element (Peltier element). For example, thebattery 9635 may be charged with a non-contact power transmission modulewhich is capable of charging by transmitting and receiving power bywireless (without contact), or another charging means may be used incombination.

In a television set 8000 in FIG. 8A, a display portion 8002 isincorporated in a housing 8001. The display portion 8002 displays animage and a speaker portion 8003 can output sound. The transistor of oneembodiment of the invention disclosed in this specification can be usedfor the display portion 8002.

A semiconductor display device such as a liquid crystal display device,a light-emitting device in which a light-emitting element such as anorganic EL element is provided in each pixel, an electrophoresis displaydevice, a digital micromirror device (DMD), or a plasma display panel(PDP) can be used for the display portion 8002.

The television set 8000 may be provided with a receiver, a modem, andthe like. The television set 8000 can receive a general televisionbroadcast with the receiver. Furthermore, the television device 8000 canbe connected to a communication network by wired or wireless connectionvia the modem, which enables one-way (from a transmitter to a receiver)or two-way (between a transmitter and a receiver, between receivers, orthe like) data communication.

In addition, the television set 8000 may include a CPU for performinginformation communication or a memory. A memory or a CPU which includesthe transistor of one embodiment of the invention disclosed in thisspecification can be used for the television set 8000.

In FIG. 8A, an air conditioner including an indoor unit 8200 and anoutdoor unit 8204 is an example of an electronic appliance to which aCPU including the transistor of one embodiment of the inventiondisclosed in this specification is applied. Specifically, the indoorunit 8200 includes a housing 8201, an air outlet 8202, a CPU 8203, andthe like. FIG. 8A illustrates the case where the CPU 8203 is provided inthe indoor unit 8200; the CPU 8203 may be provided in the outdoor unit8204. Alternatively, the CPU 8203 may be provided in both the indoorunit 8200 and the outdoor unit 8204. Since the CPU including thetransistor of one embodiment of the invention disclosed in thisspecification is a CPU using an oxide semiconductor, an air conditionerwith high heat resistance and high reliability can be achieved.

In FIG. 8A, an electric refrigerator-freezer 8300 is an example of anelectronic appliance which is provided with the CPU formed using anoxide semiconductor. Specifically, the electric refrigerator-freezer8300 includes a housing 8301, a door for a refrigerator 8302, a door fora freezer 8303, a CPU 8304, and the like. In FIG. 8A, the CPU 8304 isprovided in the housing 8301. By using the CPU including the transistorof one embodiment of the invention disclosed in this specification asthe CPU 8304 of the electric refrigerator-freezer 8300, powerconsumption can be reduced.

FIG. 8B illustrates an example of an electric vehicle which is anexample of an electronic appliance. An electric vehicle 9700 is equippedwith a secondary battery 9701. The output of the electric power of thesecondary battery 9701 is adjusted by a control circuit 9702 so that theelectric power is supplied to a driving device 9703. The control circuit9702 is controlled by a processing unit 9704 including a ROM, a RAM, aCPU, or the like which is not illustrated. By using the CPU includingthe transistor of one embodiment of the invention disclosed in thisspecification as a CPU of the electric vehicle 9700, power consumptioncan be reduced.

The driving device 9703 includes a DC motor or an AC motor either aloneor in combination with an internal-combustion engine. The processingunit 9704 outputs a control signal to the control circuit 9702 based oninput data such as data of operation (e.g., acceleration, deceleration,or stop) by a driver or data during driving (e.g., data on an upgrade ora downgrade, or data on a load on a driving wheel) of the electricvehicle 9700. The control circuit 9702 adjusts the electric energysupplied from the secondary battery 9701 in accordance with the controlsignal of the processing unit 9704 to control the output of the drivingdevice 9703. In the case where the AC motor is mounted, although notillustrated, an inverter which converts direct current into alternatecurrent is also incorporated.

The structures, methods, and the like described in this embodiment canbe combined as appropriate with any of the structures, methods, and thelike described in the other embodiments.

EXAMPLE

In this example, the crystalline state of an oxide semiconductor filmwas observed. As samples, Example Sample and Comparative Sample wereformed.

An In—Ga—Zn—O film with a thickness of 200 nm was formed as an oxidesemiconductor film over a glass substrate by a sputtering method usingan oxide target with the following atomic ratio, In:Ga:Zn=1:1:1.

The film formation conditions of Example Sample were as follows: theatmosphere was an oxygen atmosphere (oxygen=100 sccm), the pressure was0.6 Pa, the power of the power source was 2 kW, and the substratetemperature was 170° C.

The film formation conditions of Comparative Sample were as follows: theatmosphere was an atmosphere of argon and oxygen (argon:oxygen=10sccm:90 sccm), the pressure was 0.6 Pa, the power of the power sourcewas 5 kW, and the substrate temperature was 170° C.

End planes were cut out from Example Sample and Comparative Sampleobtained through the above-described process, and cross-sections of theIn—Ga—Zn—O films were observed with a high-resolution transmissionelectron microscope (TEM: “H9000-NAR” manufactured by HitachiHigh-Technologies Corporation) at an acceleration voltage of 300 kV.FIG. 1 is a TEM image of Example Sample at a magnification of 8 million,and FIG. 2 is a TEM image of Comparative Sample at a magnification of 8million.

Further, X-ray diffraction (XRD) of the In—Ga—Zn—O film of each ofExample Sample and Comparative Sample was measured. FIGS. 4A and 4B showXRD spectra of Example Sample and Comparative Sample, respectively,which were measured by an out-of-plane method, and FIG. 4B shows an XRDspectrum of Comparative Sample, which was measured by an out-of-planemethod. In FIGS. 4A and 4B, the vertical axis indicates the X-raydiffraction intensity (arbitrary unit) and the horizontal axis indicatesthe rotation angle 2θ(degree). Note that the XRD spectra were measuredwith the use of an X-ray diffractometer D8 ADVANCE manufactured byBruker AXS.

As shown in the TEM image of FIG. 1, in the In—Ga—Zn—O film of ExampleSample, a crystal part in a layer form, whose c-axis is substantiallyperpendicular to a surface of the film and whose length in a directionperpendicular to the c-axis is more than 1.5 nm was observed. Note thatin FIG. 1, the crystal part is a region surrounded by a dotted line, andthe length of a crystal arrangement part containing indium and oxygen inthe In—Ga—Zn—O film of Example Sample is the length of an arrow inFIG. 1. In the XRD spectrum of Example Sample as shown in FIG. 4A, apeak attributed to a diffraction on the (009) plane of an InGaZnO₄crystal was observed at around 31° (=2θ).

While, as shown in the TEM image of FIG. 2, in the In—Ga—Zn—O film whichis Comparative Sample, a crystal having a c-axis substantiallyperpendicular to a surface of the film was not observed. Further, asshown in FIG. 4B, a peak indicating the crystal having a c-axissubstantially perpendicular to the surface is not observed in the XRDspectrum.

The oxide semiconductor film of Example Sample includes a crystal partwhose c-axis is substantially perpendicular to the surface, and thelength of a crystal arrangement part containing indium and oxygen on aplane perpendicular to the c-axis in the crystal part is more than 1.5nm. By using such an oxide semiconductor film, a transistor havingstable electric characteristics and a highly reliable semiconductordevice can be provided.

This application is based on Japanese Patent Application serial no.2012-056643 filed with Japan Patent Office on Mar. 14, 2012, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. An oxide semiconductor film comprising: a crystalpart having a c-axis substantially perpendicular to a surface of theoxide semiconductor film, the crystal part comprising a crystalarrangement part containing indium atoms and oxygen atoms on a planeperpendicular to the c-axis in the crystal part, wherein a length of thecrystal arrangement part is more than 1.5 nm.
 2. The oxide semiconductorfilm according to claim 1, wherein the crystal part further includesgallium atoms and zinc atoms.
 3. A transistor comprising the oxidesemiconductor film according to claim 1 in a channel formation region ofthe transistor.
 4. An oxide semiconductor film comprising: a crystalpart having a c-axis substantially perpendicular to a surface of theoxide semiconductor film, the crystal part comprising a crystalarrangement part containing indium atoms and oxygen atoms on a planeperpendicular to the c-axis in the crystal part, wherein a length of thecrystal arrangement part is more than or equal to 2 nm and less than orequal to 20 nm.
 5. The oxide semiconductor film according to claim 4,wherein the crystal part further includes gallium atoms and zinc atoms.6. A transistor comprising the oxide semiconductor film according toclaim 4 in a channel formation region of the transistor.
 7. An oxidesemiconductor film comprising: a plurality of crystal parts, whereinc-axes of the plurality of crystal parts are aligned; wherein each ofthe c-axes is substantially perpendicular to a surface of the oxidesemiconductor film; and wherein in each of the plurality of crystalparts, a length of a crystal arrangement part containing indium atomsand oxygen atoms on a plane perpendicular to the c-axis is more than 1.5nm.
 8. The oxide semiconductor film according to claim 7, wherein theeach of the plurality of crystal parts further includes gallium atomsand zinc atoms.
 9. A transistor comprising the oxide semiconductor filmaccording to claim 7 in a channel formation region of the transistor.10. An oxide semiconductor film comprising: a plurality of crystalparts, wherein c-axes of the plurality of crystal parts are aligned;wherein each of the c-axes is substantially perpendicular to a surfaceof the oxide semiconductor film; wherein in each of the plurality ofcrystal parts, a length of a crystal arrangement part containing indiumatoms and oxygen atoms on a plane perpendicular to the c-axis is morethan or equal to 2 nm and less than or equal to 20 nm.
 11. The oxidesemiconductor film according to claim 10, wherein the crystal partincludes gallium atoms and zinc atoms.
 12. A transistor comprising theoxide semiconductor film according to claim 10 in a channel formationregion of the transistor.